Method and apparatus for electrochemical milling



Jan. 16, 1962 P. A. OELGOETZ 3,017,341

METHOD AND APPARATUS FOR ELECTRO-CHEMICAL MILLING Filed Jan. 21, 1960 I7WORKPIECE (ANODIC) INVENTOR.

PAUL A. OELGOETZ ATTORNEY 3,017,341 METHGD AND AkPARATUS FfiR ELECTRU-CHEMICAL MILLENG Paul A. Gelgoetz, Columbus, Qhio, assignor to NorthAmerican Aviation, Inc. Filed Jan. 21, 1960, Ser. No. 3,799 8 Claims.(Ql. 204242) This invention pertains to a method and apparatus for metalremoval, and more partcularly concerns an electrochemical method andelectro-chemical apparatus for removing metal from regions adjacentselected surface areas of steel panels and the like.

Also, this application is a coninuation-in-part of my co-pendingapplication Serial No. 665,553, filed June 13, 1957, now abandoned.

Numerous problems have been encountered in connection with attempts touniformly remove metal from selected surface areas of relatively thin,panel-like, steel aircraft components which are typically contoured toeither a flat, curved, or irregular configuration, or combinationsthereof, prior to the metal removal operation. Generally, metal removalfrom panel surface areas is difficult in that it must be to a uniformdepth with respect to the reference surface of the part, and furthermust be effected without introducing adverse stresses, as bymachine-tool workhardening, into the part.

Typical acid etching techniques have not proven acceptable for use inmilling aircraft panel components fabricated of the highly-alloyed orheat-treatable steels because such materials are especially susceptibleto hydrogen embrittlement. Similar steel parts subjected cathodically toan electrolytic acid etching treatment will also develop hydrogenembrittlement.

Conventional elcctro-chemical metal removal methods and apparatus havefurther proven deficient in that steel parts milled thereby have notbeen etched at uniform rates of metal removal throughout surface areasof substantial extent. Such unevenness in depth of metal removal isgenerally not acceptable when related to the production of aircraftcomponents having stringent weight limitations and maximum stressconditions associated therewith.

Problems have also been encountered with respect to the intergranularcorrosion introduced into certain steel workpieces when milled throughuse of known acid etching techniques. In addition, the practice ofelectro-chemical milling metal removal techniques using state-of-the-artapparatus has posed problems with respect to obtaining con sistenttolerances of small magnitude substantially throughout surface areas ofconsiderable extent. Further, the use of known methods and apparatus forthe stated purpose of electro-chemically removing steel from surfaceareas of steel panelshas required the use of more electrical energy thanis comparatively required in connection with the practice of thisinvention.

Accordingly, it is an object oflthis invention to provide a method ofand apparatus for metal removal which will result in acceptably smoothfinal surface finishes and which further will not introducework-hardening effects,

into steel parts subjected thereto.

Another object of this invention is to provide a chemical milling methodutilizing acids which will not expose highly alloyed steel partssubjected to the process to the phenomenon of hydrogen embrittlement.

Another object of this invention is to provide an electrochemicalmilling method and related tooling which are effective to remove metalfrom extended or substantial surface areas of steel materials at uniformmetal-removal rates.

Another object of this invention is to provide a metal- 3,l7,34lPatented Jan. 16, 1982 working process and related apparatus which maybe used for uniformly removing metal from pre-formed steel parts whichhave either flat, curved, or irregular surface configurations.

Another object of this invention is to provide a metalremoval method andrelated apparatus which may be advantageously utilized in connectionwith the provision of holes or clear-through openings in steelworkpieces and the like.

A still further object of my invention is to provide a method forremoving metal from ultra-high-strength steel panels which does notsubject such panels to the phenomenon of intergranular corrosion.

Another object of this invention is to provide methods and apparatus forelectro-chemically removing metal from selected regions of a steel-likepanel with a reduced requirement as to total electrical energy.

Another object of this invention is to provide methods and apparatus forelectro-chemically removing metal from steel workpieces in a mannerwhich obtains improved tolerances of greater consistency throughoutsurface areas of considerable size.

Another object of this invention is to provide a metalworking techniqueand metal-working apparatus which utilize commercially-availablematerials and equipment for operation and construction. Additionally,the equipment utilized for the process of this invention iscomparatively simple in construction, is operable by semi-skilledoperating personnel, and is free of complex maintenance difficulties.

Other objects and advantages of this invention will become more apparentwhen consideration is given to the drawings and written descriptionforming portions of this application.

In the drawings:

FIG. 1 is a perspective view of portions of the apparatus employed inthe practice of my invention;

FIG. 2 is an exploded perspective view of tooling and workpiececomponents employed in the arrangement of FIG. 1;

.FIG. 3 is a side elevational view of the components of FIG. 2 in theirassembled relation;

FIG. 4 is a partial sectional view taken along the line 4-4 of FIG. 3;

FIG. 5 is a partial sectional view taken along the line 55 of FIG. 3;

FIG. 6 is a sectional view of a typical steel panel from which metal hasbeen removed utilizing the electro-chemical methods and toolingapparatus of my invention; and

FIG. 7 is a sectional view similar to the view of FIG. 6 but showing thetypical steel panel from which metal has been removed using conventionalpractices.

The apparatus illustrated in FIG. 1 may be utilized to practice themethod of this invention and is comprised essentially of process tank10, electrolytic acid solution 11, tooling-work-piece assembly 12, andthe electrical bus bars 13 and 14. Bus bars 13 are electricallyconnected to a source of electrical energy (not shown) in cathoderelation, whereas bus bar 14 is connected to the same source ofelectrical energy in anode relation. Such source of electrical energypreferably provides direct-type current and may take the form of asource of rectified alternating current electrical energy.

Process tank 10 is preferably constructed of, or lined with, a materialthat is chemically-resistant to the acid solution 11; contained therein.Materials such as wood, polyvinyl chloride, natural or synthetic rubber,organic materials such as polytetrafluoroethylene, and the like haveproved acceptable and are well known to those familiar with the art.

Acid solution 11 is electrically conducting, typically containsdisassociated hydrogen ions, and chemically reacts with thehereinafter-described workpiece contained in tooling assembly 1. Anetching solution which contains 25%i-5% sulfuric acid, by weight, ispreferred in the practice of my invention. However, it is recognizedthat other single acids, such as phosphoric acid, and combinations ofacids will prove adequate when substituted for sulfuric acid. It isbelieved that minimum maintenance and control is required when singleacid solutions are employed. In practicing this invention it isdesirable that the operating temperature of solution 11 be maintained atapproximately 80 F .110" F., and that the solution be continuouslycirculated as by air agitation.

FIG. 2 illustrates assembly 12 as including cathode member 15, anodeframe member 16, and partiallycoated steel panel workpiece 17. Ametallic bleeder member 18 is connected to frame member 16 by means offastener devices (rivets) 20 in electrically joined relation. Similarly,fastener devices 19 are utilized to connect workpiece 17 to frame member16 in electrically joined relation. Additional details regarding cathodemember 15 and the components of frame member 16 and bleeder member 18Will be provided hereinafter.

Steel panel 17 is illustrated in the drawings as having a contour whichis curved in one direction and is provided with a chemically-resistantdielectric strippable coating 22 in adhering relation to surface areaswhich are not to be subjected to the electro-chemical milling method ofthis invention. In preparing panel 17 for assembly to frame member 16 itis preferred that the contoured workpiece first be cleaned usingconventional degreasing equipment or the like. It is also preferred thatthe steel workpiece be subjected to a sand blasting treatment in orderthat uniform adhesion of subsequently applied strippable coating 22 beobtained. A surface roughness of approximately 100 micro inches R.M.S.generally proves entirely satisfactory for this purpose. Themanually-strippable coating 22 is preferably next applied over allsurfaces of the panel through use of a spraying or dipping technique.Materials suitable for the coating include modified vinyl polymers,cellulose acetate butyrate, polyethylene, and the like. One particularlyeffective maskant material is compounded of approximately 18% polyvinylchloride, 2% cellulose acetate butyrate modifier, and approximately 80%solvents such as methyl isobutyl ketone and methyl ethyl ketone, allpercentages being by weight. Adequate chemical and electrical resistanceis generally obtained by applying two wet cross-coats of the material tothe surface of panel 17. Sufficient time should be allowed between theapplication of the various maskant coats to permit the solventscontained therein to suificiently evaporate. The final thickness ofmanually-strippable coating 22 generally approximates .002. If required,a comparatively high-temperature baking step may be necessary to obtaincomplete polymerization of the maskant.

In continuing the preparation of steel panel 17, prior to assembly totooling component 16, a template and heated stylus are utilized toscribe the maskant edge 21 in coating 22. Edge 21 defines the peripheryof the panel surface area from which metal is to be removed by thepractice of this invention. As shown in FIG. 4, metal may be typicallyremoved to the extent of the dotted line showing. Such removal resultsin undercutting with respect to edge 21 and, accordinggly, edge 21 islocated only to within one-half to once the depth of etch of the desiredmilled area outline. The interior, to-be-etched, surface area of panel17 may be made ready for electrochemical milling simply by manuallystripping those portions of maskant 22 which are located within theoutline of scribed edge 21. Also, portions of coating 22 are removedfrom the reverse side of panel 17 at the areas designated 23 in orderthat metallic fastener means 19 might electrically connect panel 17 tothe rectangular portion 2 of frame member 16. See FIG. for anillustration of this detail.

Frame assembly 16 includes generally rectangular frame portion 24 andthe hook members 25 connected thereto, as by a weld. Members 25 areconnected to frame portion '24 in electron-conducting relation and areshaped to cooperate with anode bus bar 14. Also, frame portion 24 isprovided with integral lugs 26 for attaching panel 17 thereto. Frameportion 2 4, hook members 25, and lug members 26 are preferablyfabricated from a Weldable aluminum alloy type of material, titanium, orthe like. As shown in FIG. 4, an electro-chernically resistant maskant27 is provided in adhering relation to exposed surface areas of frameassembly 16 which will be typically immersed in acid solution 11.However, coating 27 is preferably applied after bleeder member 18 isoperatively connected to fram assembly 16.

Bleeder member 18 is conveniently fabricated of an electron-conductingmaterial, such as lead, and is comprised of a rectangular portion havingan open area which conforms generally to the outline of the scribed edge21 provided in coating 22. The interior edge of member 18 is, however,preferably located outwardly with respect to scribed edge 21 a slightdistance. Tab portions 28 of member 18 extend outwardly with respect tothe bleeder interior opening, are recessed in frame portion 24, andestablish electrical contact with frame member 16 through an abuttingsurface-to-surface relation and through the use of metallic fastenermeans 20. Bleeder member 18 is preferably not provided with a protectivecoating or maskant in its various exposed surface areas. Bleeder member18 is, however, provided with a contour that corresponds to the contourconfiguration of workpiece 17.

Cathode member 15 is provided with the following construction. Platecomponent 29 has hook member 30 connected thereto and is provided with amultiplicity of individual cathode elements 31. Hook member 30 is shapedto conform to a cathode bus bar 13. As indicated in FIGS. 4 and 5,cathode elements 31 are inserted in openings 32 and are attached toplate 29 by the welds designated 33. Plate 29, hook 30, and cathodeelements 31 are also preferably fabricated of a weldabl aluminum alloymaterial, titanium, or the like. By providing plate 29 with a contourwhich generally conforms to the curvature of workpiece 17, I am able tolocate the end portions 34 of spaced-apart cathode elements 29 a uniformdistance from uncoated surface area within scribed edge 21 when cathodemember 15 is secured to frame 16.

Cathode member 15 is attached to frame member 16 by the fastenerarrangement illustrated in FIG. 4. Bolt member 35 is threaded atopposite ends and is engaged with a threaded opening provided in frameportion 24. Insulating spacer members 36 and 37 are provided withshoulders that cooperate with plate 29 and serve to separate plate 29from bolt 35 and from the frame assembly in electron-conductingrelation. Nut device 38 cooperates with the free threaded end of boltmember 35 and retains the cathode member 15 in proper relation to framemember 16 and the workpiece panel 17 assembled thereto. In the practiceof this invention for production purposes, it is necessary only that theindividual workpiece panels be attached to and removed from the toolingassembly as complete disassembly of the apparatus described herein isnot required.

The following detailed information is provided to describe the specificapparatus which has been employed to mill a fiat, uncoated panel surfacearea 5 /2 x 13 /2" to a depth of 0.036" in a panel of PH17-7 MO annealedsteel 0.042"-thick. Frame member 16 and cathode member 15 werefabricated of Type 618 aluminum alloy. Cathode elements 31 werefabricated of diameter rod material, were located in openings spaced incollector plate 29 at 1 on centers, and were welded to project from theface of plate 29 approximately 1". Lead bleeder member 18 was providedwith a cross-section of A x A" and was located with its interior edgespaced laterally outwardly A from the scribed edge 21 provided in theworkpiece. In assembling cathode member 15 into tooling assembly 12, theends 34 of the various cathode elements were maintained an approximatedistance of 1" from the uncoated panel surface area. The line of centersof the outermost rows of cathode elements were located laterally withinthe uncoated panel area to within approximately A of the interiorscribed edge 21. Hence, the outermost portions of elements 31 and theinterior edge of bleeder member 18 were laterally spaced apartapproximately The tooling assembly was placed in the heretoforedescribedsulfuric acid solution 11 in processing tank so that hook components 25cooperated with anode bus bar 14 and hook member 30 of cathode 15cooperated with an electrode 13. When components 13 and 14 wereconnected to a source of electrical energy of approximately 8 voltspotential, a circuit was established whereby a direct electrical currentwas conducted from bus bar 14, into hook member 30 and plate 29 to theindividual cathode elements 31, through solution 11, and simultaneouslyinto workpiece panel 17 and bleeder member 18. The electrcal currentflowing into bleeder member 18 was conducted therefrom into frameportion 24, partially through fastener means 20. The direct electricalcurrent flowing across the uncoated surface of workpiece 17 passed fromworkpiece 17 through fastener devices 19 and into frame portion 24.Electrical current passing into frame portion 24 was then conductedthrough hook members 25 and into anode bus bar 14 to thereby completethe electrical circuit. The current density maintained across suchworkpiece was approximately 8 amperes per square inch of to-be-milledsurface. The above-described operation of apparatus components removedmetal from the workpiece in a most effective manner. See FIG. 6 for anillustration of the crosssection configuration which typically resultsfrom the practice of this invention; as shown therein,electro-chemically milled surface 41 of panel 40 is flat with respect tothe underside reference surface of that panel. Dimensional tolerances ofthe PHl7-7 MO steel part as to uniform thickness were within plus orminus 0.001". Surface roughness of the milled area was micro inches RMSor less, and neither hydrogen embrittlement nor intergranular corrosioncharacteristics were present in the finished product.

For illustration purposes I have provided a FIG. 7 which shows thecrowned condition which is present in surface area 43 when a panel 42 ismilled using conventional electro-chemical milling techniques andapparatus. Such crowned condition, together with the accompanyingdimensional variations are generally not acceptable for aircraft-qualitypanel components. Such disadvantages are effectively overcome throughthe practice of this invention.

During the process, oxy en is given off at the interface between theuncoated surface area within coating edge 21 and electrolyte solution11, and hydrogen gas 1s evolved at cathode member 15. The chemical rection between the solution and the workpiece panel 17 produces the ironsalt of the solution acid and results in the removal of metal. Theprocess must be continued a sufficient time to remove metal to thedesired depth. such as to the dotted line position indicated in FIG. 4.To completely avoid hydrogen embrittlement, electrical current should beflowed to and from tooling assembly 12 as it is being both immersed inand removed from solution bath 11.

It is to be understood that the form of the invention herewith shown anddescribed is to be taken as the preferred embodiment of the same;various changes in shape, size, and arrangement of component parts, asfor removing metal from workpiece panels of different configuration. maybe effected without departing from the spirit of the invention or fromthe scope of the claims.

I claim:

1. An assembly for use with an electrolyte solution and with anelectrical energy supply to remove metal from an included workpiece, andcomprising: said workpiece having an uncoated surface area, an anodeframe having an interior opening, bleeder means contoured to cooperatewith said workpiece and having an interior opening that encircles saidworkpiece uncoated surface area and that is located within said anodeframe interior opening, cathode plate means, rod-like cathode elementsconnected to said plate means and having outward end portions which areprojected to conform to said workpiece uncoated surface area as toprojected contour, first fastener means securing said bleeder means tosaid frame fixedly and in electronconducting relation, second fastenermeans securing said cathode plate means to said frame fixedly and inelectrically-insulated relation, and third fastener means securing saidworkpiece to said frame fixedly and in electron-conducting relation,said third fastener means locating said workpiece so that said bleedermeans is spacedapart a uniform distance from said workpiece uncoatedsurface area and from said cathode element outward end portions.

2. Apparatus for removing metal to a uniform depth throughout a paneluncoated inset surface area and combined with a workpiece panel having acoating edge which defines said inset surface area, comprising: anelectrical energy supply, electrolyte solution contained in a processtank means and having said uncoated inset surface area immersed therein,anode frame means electrically connected to said workpiece panel and tosaid electrical energy supply, a multiplicity of rod-like cathodeelement means connected to said electrical energy supply and projectedtoward and spaced apart from said uncoated inset surface area uniformlythroughout the normal projection of said uncoated inset surface area,and bleeder means electrically connected to said electrical energysupply and having an interior opening edge which encircles said coatingedge, said bleeder means interior opening edge being uniformly spacedapart from the laterally outermost of said cathode element means.

3. The apparatus-workpiece panel combination defined in claim 2, whereinsaid bleeder means interior opening edge is spaced laterally outwardlywith respect to said coating edge a uniform distance throughout theperiphery of said workpiece inset surface area, said bleeder meansinterior opening edge being uniformly spaced apart from the laterallyoutermost of said cathode element means and from said coating edge.

4. Apparatus for use in electro-chemically removing metal from aworkpiece having an exposed inset surface area which is defined by acoating edge, and comprising: an electrical energy supply, frame meansfor supporting said workpiece, means anodically connecting said framemeans to said electrical energy supply, a grouping of rod-like cathodeelements arranged over an area which is coextensive with said workpieceexposed inset surface area, means cathodically connecting said rod-likecathode elements to said electrical energy supply, electron-conductingbleeder means having an interior opening which encircles said workpieceexposed inset surface area, means securing said rod-like cathodeelements, said bleeder means, and said workpiece in fixed relationrelative to said frame means and relative to each other, and processtank means having an electrolyte solution which receives said rod-likecathode elements, said bleeder means, and said workpiece inset surfacearea to remove metal from said workpiece, said bleeder means beingelectrically connected to said frame means. and said workpiece beingelectrically connected to said frame means and positioned so that saidexposed inset surface area is aligned with said bleeder means interioropening and is aligned with and spaced-apart a uniform distance fromsaid rod-like cathode elements.

5. The apparatus defined in claim 4, wherein said b eeder means interioropening has an edge which is similiarly shaped and oversized withrespect to the coating edge that defines said workpiece exposed insetsurface area, said bleeder means interior opening edge being spaced fromsaid workpiece coating edge a uniform distance around the periphery ofsaid workpiece exposed inset surface area.

6. The apparatus defined in claim 4, wherein said bleeder means interioropening has an edge which is similarly shaped and oversized with respectto the coating edge that defines said workpiece exposed inset surfacearea, said bleeder means interior opening edge being spaced-apart fromthe outline extreme of said grouping of rod-like cathode elements auniform distance around the periphery of said workpiece exposed insetsurface area.

7. The apparatus defined in claim 4, wherein fastener means are providedfor securing said bleeder means to said frame means and for securingsaid workpiece to said frame means, said workpiece-securing fastenermeans being removable from said frame means separately from said bleedermeans-securing fastener means.

8. A tooling assembly for use with an electrolyte solution and with asource of electrical energy to remove metal from a workpiece panelcombined with said tooling assembly and having an uncoated surface areaand having a coating edge which defines said uncoated surface area, andcomprising: an anode frame member having an interior opening, bleedermeans contoured to conform to the contour of said workpiece panel andhaving an interior opening edge which is located within said framemember interior opening and which encircles said relation to said framemember interior opening, first fastener means fixedly securing saidbleeder means to said frame member in electron-conducting relation,separate fastener means fixedly securing said cathode means to saidframe member in electrically insulated relation, and other fastenermeans fixedly securing said workpiece panel to said frame member inelectron-conducting relation, said other fastener means locating saidworkpiece panel relative to said frame member so that said workpieceuncoated surface area is positioned adjacent to and encircled by saidbleeder means interior opening edge and is spaced-apart from saidcathode means electrical energy distribution elements a uniform distancethroughout the extent of said workpiece uncoated surface area.

References Cited in the file of this patent UNITED STATES PATENTS1,376,365 Wertheimer Apr. 26, 1921 2,539,502 Zanetti et a1 Jan. 30, 19512,739,935 Kehl et a1 Mar. 27, 1956 2,844,531 Prince July 22, 1958FOREIGN PATENTS 901,118 France -e- July 18, 1945

1. AN ASSEMBLY FOR USE IN WITH AN ELECTROLYTE SOLUTION AND WITH ANELECTRICAL ENERGY SUPPLY TO REMOVE METAL FROM AN INCLUDED WORKPIECE, ANDCOMPRISING: SAID WORKPIECE HAVING AN UNCOATED SURFACE AREA, AN ANODEFRAME HAVING AN INTERIOR OPENING, BLEEDER MEANS CONTOURED TO COOPERATEWITH SAID WORKPIECE AND HAVING AN INTERIOR OPENING THAT ENCIRCLES SAIDWORKPIECE UNCOATED SURFACE AREA AND THAT IS LOCATED WITHIN SAID ANODEFRAME INTERIOR OPENING, CATHODE PLATE MEANS, ROD-LIKE CATHODE ELEMENTSCONNECTED TO SAID PLATE MEANS AND HAVING OUTWARD END PORTIONS WHICH AREPROJECTED TO CONFORM TO SAID WORKPIECE UNCOATED SURFACE AREA AS TOPROJECT CONTOUT, FIRST FASTENER MEANS SECURING SAID BLEEDER MEANS TOSAID FRAME FIXEDLY AND IN ELECTRONCONDUCTING RELATION, SECOND FASTENERMEANS SECURING SAID CATHODE PLATE MEANS TO SAID FRAME FIXEDLY AND INELECTRICALLY-INSULATED RELATION, AND THIRD FASTENER MEANS SECURING SAIDWORKPIECE TO SAID FRAME FIXEDLY AND IN ELECTRON-CONDUCTING RELATION,SAID THIRD FASTENER MEANS LOCATING SAID WORKPIECE SO THAT SAID BLEEDERMEANS IS SPACED APART A UNIFORM DISTANCE FROM SAID WORKPIECE UNCOATEDSURFACE AREA AND FROM SAID CATHODE ELEMENT OUTWARD END PORTIONS.